1. Field of the Invention
This invention relates generally to a coherent demodulating arrangement for use in a digital radio communications system, and more specifically to such a demodulating arrangement by which a high quality baseband signal can be obtained even if a modulated incoming signal deviates in frequency. This invention is directed to an arrangement for demodulating the signals which have been PSK modulated.
2. Description of the Prior Art
As is known in the art, coherent demodulating arrangements are used to detect a baseband signal through multiplication of a modulated incoming IF (Intermediate Frequency) signal and a reproduced carrier signal. The reproduction of a carrier signal is implemented by a phase-locked loop using a demodulated baseband signal.
Before describing the present invention in detail, reference will be made to two known coherent demodulating arrangements with reference to FIGS. 1 and 2, respectively.
In FIG. 1, an incoming modulated IF signal is applied via an input terminal 10 to a non-coherent demodulator 12, which is coupled to a local oscillator 14 via a phase shifter 11 and a signal distributer 13. The phase shifter 11 shifts, by .pi./2 radians, the phase of the output of the local oscillator 14 applied to the non-coherent demodulator 12. The frequency of the output of the oscillator 14 is previously set to the frequency of the incoming modulated IF signal. The term "non-coherent" in this specification implies that there exists no frequency or phase synchronization between the local oscillator 14 and the received IF signal. The output of the non-coherent demodulator 12 is, in most cases, not equal to a baseband signal but is similar thereto, and is applied to a coherent demodulator 16. The coherent demodulator 16 forms part of a phase-locked loop 18 which, in addition to the coherent demodulator 16, comprises a channel filter 20, a phase detector 22, a loop filter 24 and a voltage controlled oscillator (VCO) 26. As shown, the VCO 26 is coupled to the coherent demodulator 16 via a signal distributor 23 and a phase shifter 25. These blocks 23, 25 exhibit the same functions as the blocks 11, 13, respectively. The two lines between the blocks 12, 16 are provided for signals having complex values, that is having a real part and an imaginary part. This applies to the two lines between the blocks 16, 20 and also between the blocks 20, 22. A phase-locked loop is itself well known in the art and hence the details thereof will not be described for brevity.
The coherent demodulator 16 receives a reproduced carrier wave from the VCO 26 and performs multiplication between the output of the unit 12 and the reproduced carrier wave. The output of the coherent demodulator 16 takes the form of a baseband signal which is applied to the phase detector 22 via the channel filter 20 which in this case exhibits a low-pass characteristics. The output of the coherent demodulator shown in FIG. 1, takes the form of a signal having a complex value and is derived from the channel filter 20. Since the coherent demodulator 16 precedes the channel filter 20, there is no degradation of signal wave even in the case of frequency deviation of incoming IF signal due to doppler shift in a satellite communications system, for example. However, the FIG. 1 arrangement has encountered the problem that the frequency response of the phase-locked loop 18 is undesirably lowered due to delay of signal transmission in the channel filter 20.
In order to overcome this problem, a second prior art demodulating arrangement, shown in FIG. 2, has been proposed. The arrangement of FIG. 2 is basically identical to that of FIG. 1 and differs only in that the coherent demodulator 16 is preceded by the channel filter 20. This overcomes the aforesaid lowering of the response of the phase-locked loop. However, since the frequency of the local oscillator 14 is fixed to that of the incoming IF signal, when a frequency deviation occurs in the incoming IF signal, the output of the non-coherent demodulator 12 is apt to contain frequency components which are out of the passband of the filter 20. This induces the situation wherein the wave shape of the output signal of the channel filter 20 is degraded and causes intersymbol interference. As a result, the prior art shown in FIG. 2 cannot be relied upon to provide high quality demodulated signals under all circumstances.
It should be noted that it would be possible for the local oscillator frequency to be equal to the frequency of the incoming IF signal although such a case is rare with a satellite communications system (for example). In which case, the VCO 26 terminates oscillation if the output of demodulator 12 (viz., baseband signal) continues to be applied to the demodulator 16 together with a reproduced carrier from the VCO 26. That is to say, the output of the VCO 26 becomes a direct current signal. Hence, the coherent demodulator 16 no longer functions as a demodulator. In other words, the circuit operates as if the output of the demodulator 12 is directly connected to the channel filter (FIG. 1) or to the phase detector 22 via the channel filter (FIG. 2).